This invention relates in general to induction motor drive systems and, more particularly, to an induction motor drive system for operating a single-phase, two winding induction motor from a two-phase power supply circuit.
A conventional split-phase capacitor start or capacitor run single phase induction motor, also known in the art and referred to hereinafter as a permanent split capacitor (PSC) motor, has two stator windings, a "main" winding and a "start" winding. FIG. 1 illustrates an exemplary PSC motor 100 that includes a main winding 102 and a start winding 104 that are commonly connected at one end. Main winding 102 and start winding 104 are mounted in the stator (not shown) of motor 100 and spatially separated from each other by an angle related to the rated speed of motor 100, e.g., 90.degree. for a two pole, 3600 RPM motor, as is well known in the art. Such PSC motors are designed to be operated with a run capacitor, such as a run capacitor 106, connected in series with start winding 104. It is a typical practice in the industry for the motor manufacturer to not supply the run capacitor with the motor, but instead to only specify parameters of the capacitor, e.g., capacitance and power rating, sufficient to enable a user to procure and install the capacitor.
In the operation of PSC motor 100, main winding 102 and the series combination of start winding 104 and run capacitor 106 are connected in parallel with each other and directly across a single phase power source 110. Since start winding 104 is energized through capacitor 106, the phase angle of the current flowing through start winding 104 is shifted with respect to the current flowing through main winding 102, such that the phase angle between the respective currents flowing in windings 102 and 104 is 90.degree. while the motor is running. The phase angle between the currents in windings 102 and 104 and the spatial separation of those windings result in the creation of a rotating magnetic field which is inductively coupled to the rotor (not shown) of motor 100, to exert a rotational force on the rotor.
The rotor of motor 100 attempts to rotate in synchronism with the rotating magnetic field but lags the rotating magnetic field by a "slip" factor, resulting in a torque on the rotor which is in part proportional to the amount of slip.
The starting torque exerted on the rotor of motor 100 during a starting period when motor 100 is started and accelerated to rated speed is also proportional to the sine of the phase angle between the currents flowing in windings 102 and 104. Therefore, in order to maximize the starting torque, it is necessary to achieve a phase angle of 90.degree. during starting. However, the starting torque for a single phase PSC motor, such as motor 100, is generally poor because the specified parameters of the run capacitor are only optimized for running conditions, not starting conditions. Thus, the capacitance of run capacitor 106 is specified by the manufacturer based on the impedances of windings 102 and 104 that will be experienced during running of motor 100, rather than during starting. However, as known in the art, the apparent values of motor winding impedances vary during the starting period of a PSC motor and are therefore different during starting than during running. As a result of the capacitance of capacitor 106 being optimized for running and not for starting, its magnitude is too small for starting. This results in the phase angle between the currents flowing in windings 102 and 104 being less than 90.degree. during the starting period and the starting torque being less than a maximum possible starting torque.
One solution known in the art to compensate for the insufficient magnitude of capacitor 106 during starting is to connect a starting capacitor 112 across capacitor 106 to increase the total capacitance in series with start winding 104 and thereby increase the current flowing in the start winding, the phase angle and the starting torque of motor 100. Starting capacitor 112 is disconnected, e.g., by means of a centrifugal switch, positive temperature coefficient thermistor (PIC device) or relay, once the motor has reached running speed. Disadvantageously, although operation of starting capacitor 112 generally improves the starting torque of motor 100, its use still does not maximize torque throughout the starting period of motor 100. Ideally, the magnitude of the capacitance in series with start winding 104 would have to be continuously varied during the start period to maintain a desired phase angle while the respective impedances of windings 102 and 104 vary.
Conventional single phase PSC motors are commonly used in heating, ventilating and air-conditioning (HVAC) systems to drive system loads such as fans, pumps and compressors. HVAC systems are subject to widely varying demand cycles due to a variety of factors such as, for example, daily and seasonal fluctuations of ambient temperature, human activity in the controlled environment, and intermittent operation of other equipment in the controlled environment. Accordingly, in order to assure a satisfactory temperature of the controlled environment, the HVAC system must have the heating and/or cooling capacity to accommodate "worst case" conditions. As a result, under less than worst case conditions the HVAC system has a significant over-capacity and is necessarily operated at reduced loading. Since the maximum operating efficiency of a motor, such as a PSC motor, is normally obtained only when the motor is operating at full load, the reduced HVAC system load results in inefficient operation of the motor. Further, to the extent that motors are required to cycle on and off to meet HVAC load requirements less than the capacity of the HVAC system, further significant operating inefficiencies are experienced. Such further inefficiencies include the operating cost of frequent starting of motors as well as a reduction in useful life of such motors resulting from the well known thermal and mechanical stresses experienced during starting.
A solution for overcoming the above inefficiencies resulting from the excessive capacity of an HVAC system is to vary the system capacity to meet the demand on the system. One method for varying HVAC system capacity is by varying the speed of the motors driving the HVAC system loads in accordance with the demand. With respect to HVAC system loads driven by single phase motors, such as PSC motors, in order to effect a desired motor speed control, it would be necessary to vary the frequency of the single phase power supplied to the motor. However, with respect to PSC motors, the run capacitor, e.g., capacitor 106 of motor 100 (FIG. 1), is optimized for a particular set of running conditions, including operation at a nominal frequency, e.g., 60 Hz. As a result, operation of a PSC motor at other than the nominal frequency results in production of less than optimal torque and inefficient operation. While some applications may exist in which very limited speed control of a PSC motor is achieved by a small variation of the single phase source frequency, such variation from the nominal frequency results in less efficient operation since the motor is nevertheless designed for optimum performance at the nominal source frequency.
A conventional implementation of varying motor speed to modulate HVAC system capacity typically requires a two- or three-phase motor supplied with two- or three-phase power, respectively. The use of such polyphase motors and power supplies enables variation of motor speed by varying the frequency of the voltage applied to the motor while maintaining a constant volts/frequency (volts/hertz) ratio. Maintenance of a constant volts/hertz ratio corresponds to maintenance of a constant air gap flux and efficient motor operation while delivering rated torque. The use of polyphase motors also offers several other advantages over that of a single phase motor such as, for example, lower locked rotor currents, higher starting torque, lower full load currents and improved reliability due to elimination of the start and/or run capacitor which are required in single phase motors. Disadvantageously, such polyphase motors are more expensive than single phase motors having the same horsepower rating.
Such applications employing polyphase motors generally require provision of variable frequency polyphase power from either a single phase or polyphase line source by means of a power supply circuit, including a polyphase inverter, coupled between the motor and the line source. One drawback to this arrangement occurs in the event that the power supply circuit fails and it is not possible to connect the polyphase motor directly to the line source, such as, for example when a three-phase motor is driven by an inverter which receives power from a single phase line source. Failure of the power supply circuit therefore results in failure and unavailability of the system utilizing the polyphase motor.
Previous attempts to address the problem of backup power for polyphase motors fed from a single phase power source have required inverter redundancy or additional circuit means for temporarily directly connecting the polyphase motor to the single phase power source. However, the additional circuit means required to "simulate" polyphase power may not provide truly polyphase power and therefore may not drive the polyphase motor at optimum efficiency.